The present invention is generally related to a printhead for an inkjet printer and more particularly related to the design of ink feed channels for the ink firing chambers within the printhead to increase the speed of printing and reduce crosstalk. The present invention is related to U.S. patent application Ser. No. 08/282,243 for "Inkjet Printhead with Tuned Firing Chambers and Multiple Inlets" filed on behalf of Peter M. Burke et al. on the same date herewith.
Thermal inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles or orifices in a surface held in proximity to a medium upon which marks or printing is to be placed. These nozzles are arranged in a fashion in the surface such that the expulsion of a droplet of ink from a determined number of nozzles relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the substrate or the medium and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of nozzles so that selected firing of the orifices can produce a multicolored image by the inkjet printer.
Speed of printing (droplet ejection rate) and quality of print are essential to the user of an inkjet printer. Other factors such as spurious ink spray reduction and accurate positioning of the drop on the medium are also important.
Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent and creates a vapor phase bubble of ink. Rapid heating of the ink is achieved by passing a square pulse of electric current through a resistor, typically for 1 to 3 microseconds. Each nozzle is coupled to a small unique ink firing chamber filled with ink and having the individually addressable heating element resistor thermally coupled to the ink. As the bubble nucleates and expands, it displaces a volume of ink which is forced out of the nozzle and deposited on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of ink feed channels.
After the deactivation of the heater resistor and the expulsion of ink from the firing chamber, ink flows back into the firing chamber to fill the volume vacated by the ink which was expelled. It is desirable to have the ink refill the chamber as quickly as possible, thereby enabling very rapid firing of the nozzles of the printhead. Rapid firing of the nozzles, of course, results in high speed printing. When a drop is ejected from a printhead nozzle, some of the remaining fluid remains as an oscillating meniscus in the nozzle and replacement ink flows into the ink firing chamber to replenish ink lost to the ejected droplet. Before the next firing of the nozzle, the ink meniscus must come to rest and the ink must have sufficient time to refill the chamber, otherwise an undesirable variation in droplet weight will occur and the resultant printing will experience serious degradation of print density.
Since droplet weight must be of a predictable and uniform size for good quality printing, and because the droplet weight is generally related to the frequency at which the nozzles are fired, the printhead nozzle ejection frequency is limited. At high rates of droplet production, the time available for firing chamber refill is less. If the refill time is too short, a droplet of too low a droplet weight will be expelled in the next firing.
A large open fluid coupling between the supply of ink and the ink firing chamber would fulfill the need for high speed refilling. There are two issues for concern. First, the ink oscillation in the nozzle must be damped to minimize the variations in drop volume which will affect print quality. Second, in a practical printhead where a plurality of nozzles and firing chambers exist, such a large coupling would result not only in ink being forced from the nozzle which is being fired but also being forced via the ink feed supply route to neighboring ink firing chambers and their associated nozzles. This phenomenon is commonly referred to as crosstalk, and produces imprecisely defined characters in the printed output as a result of ink leaking from nozzles adjacent a fired nozzle and forming a puddle of ink on the external surface of the orifice plate. Thus, some form of buffering of the common ink source is necessary to prevent crosstalk between adjacent ink firing chambers. See, for example, U.S. Pat. No. 4,882,595.
It is generally accepted that good quality of graphic images require a printer resolution of 300 dpi or higher. As stated above, when one achieves high frequency of droplet ejection, one must also achieve droplets of sufficient volume to give good print quality. Smaller droplet weight results in degraded print quality. Droplet volume can also be increased by changing the resistor size or the nozzle size. These parameters in turn affect the refill rate, damping which limits the maximum frequency at which the printhead nozzle can operate. It follows, then, that in order to achieve desired droplet weight at high frequency, the ink feed channel dimensions, the orifice dimensions, and the heater resistor dimensions are critical.
It would be desirable, therefore, to realize an inkjet printhead having an increased print speed and sufficient uniform droplet weight with minimum crosstalk between neighboring ink firing chambers and nozzles.